The present invention relates to a high density semiconductor device, in particular a semiconductor device comprising charge-pump-memory cells and a process for producing the same.
The term charge-pump-memory cell used herein designates a kind of semiconductor memory cells, wherein a semiconductor region under a gate of a MOS transistor is in a floating state and information is stored in the MOS transistor as a potential variation of the floating region. The information is read out of the charge-pump-memory cell as a change in drain current due to a substrate bias effect.
Conventionally, a silicon gate MOS transistor has been used in the charge-pump-memory cell and a polycrystalline silicon layer and an aluminum layer have been used as a word line and a bit line, respectively. A cross sectional view of an embodiment of the conventional charge-pump-memory cell is shown in FIG. 1.
Referring to FIG. 1, a silicon dioxide layer 4 is situated on a sapphire substrate 1, p type silicon epitaxial region 3 is also situated on the sapphire 1 and it is in an electrically floating state thus, the p type silicon epitaxial region is called a floating substrate. On the silicon dioxide layer 4, a phosphosilicate-glass layer 5 is formed, while covering a polycrystalline silicon layer 6 forming a word line. The silicon dioxide layer 4 and the phosphosilicate-glass layer 5 are provided with a contact hole exposing the surface of the n.sup.+ type silicon epitaxial region 2. Aluminum wiring 7 is provided on the phosphosilicate glass layer 5 and in the contact hole 8 to make contact with the n.sup.+ type silicon epitaxial layer 2.
In the conventional charge-pump-memory device having such a contact hole, it is necessary to provide a certain distance between the contact hole 8 and the polycrystalline silicon layer 6, because there is a possibility that the polycrystalline silicon layer 6 and the aluminum wiring 7 will come in contact with each other. Providing the certain distance between the contact hole 8 and the polycrystalline silicon layer 6 deteriorates the packing density which is an important property of the memory device. Furthermore, when the contact hole is formed in the conventional charge-pump-memory cell, it is necessary to exactly align the position of the contact hole 8 with the position of the n.sup.+ type epitaxial layer 2 and the position of aluminum wiring 7. Additionally, the structure of the conventional memory device is very complicated, i.e. the device consists of three layers, an epitaxial single-silicon layer, a polycrystalline silicon layer and an aluminum wiring layer and further comprises the above-mentioned contact hole. Thus the process for producing the charge pump memory cell is also complicated.